This application pertains to the art of valves and, more particularly, to sequential valves which sequentially close and open ports for connecting a chamber to different pressure sources. The invention is particularly applicable to power brake units and will be described with reference thereto. However, it will be appreciated that the invention has broader aspects and may be used for devices other than power brake units.
One well-known type of power brake unit is commonly referred to as a vacuum power brake unit, and comprises a housing enclosing front and rear chambers on opposite sides of a movable wall. The front chamber is connected with the engine vacuum and communicates with the rear chamber through a valve mounted on the movable wall which is normally biased to a rest position by a spring. Operation of the brake pedal moves the valve to close communication between the chambers and opens the rear chamber to atmospheric pressure which causes movement of the movable wall due to the pressure differential in the two chambers. The movable wall carries a push rod which acts on a hydraulic cylinder for operating wheel brake cylinders.
In vacuum brake units of the type described, adequate force can be obtained if the vacuum in the front chamber is sufficiently below atmospheric pressure, and if the area of the movable wall is sufficient to obtain the desirable force due to the pressure differential in the two chambers. However, due to factors such as safety and environmental regulations, modern automobiles must have decreased stopping distances, and also result in automobiles having reduced engine vacuum levels, reduction in underhood packaging space and increased curb weight. These factors require an increase in the energy available in power brake units, while decreasing the overall size. Supercharging a vacuum power brake unit with positive air pressure is one answer to the problems.
U.S. Pat. No. 3,780,620 issued Dec. 25, 1973, to Gardner discloses one arrangement of a supercharged vacuum power brake unit. In the Gardner unit, the rear chamber is sequentially connected to vacuum, atmospheric pressure and then superatmospheric pressure. However, releasing the brake pedal connects the rear chamber directly from superatmospheric pressure to front chamber vacuum so that all of the braking force is lost. In situations where it appears desirable to have an initial high braking force and then a reduced braking force, the Gardner unit cannot operate to connect the rear chamber from superatmospheric pressure to atmospheric pressure. Instead, the Gardner unit operates to connect the rear chamber directly from superatmospheric pressure to front chamber vacuum so that all of the braking force would be lost momentarily and the brake pedal would have to be reapplied for connecting the rear chamber from atmosphere to superatmospheric pressure. The front vacuum chamber is connected with the engine intake manifold through a check valve so that the vacuum chamber captures the highest vacuum level produced by the engine and holds this level in the engine off mode so that booster output capacity is maintained on a degrading basis. In the Gardner unit, the output capacity of the booster would be zero when the front chamber is connected directly to the superatmospheric pressure in the rear chamber. It would be desirable to have a unit which would sequentially connect the rear chamber with superatmospheric pressure, atmospheric pressure and then vacuum.
The Gardner unit also requires four valves, and opening or closing movement of certain valves is not directly related to closing or opening movement of adjacent valves. This arrangement requires extremely close tolerance in order to insure substantially simultaneous opening of a next valve with closing of a previous valve. Tolerances of this nature are virtually impossible to maintain, and results in a unit having a certain amount of slack. That is, the unit must be designed so that the brake pedal will travel a short distance after closing of one valve before the next valve is opened. This is an undesirable arrangement because it does not provide smooth operation.
The Gardner unit also requires a separate passage in the valve body for communicating superatmospheric pressure with the rear chamber, while vacuum and atmospheric pressure communicate with the rear chamber through a different valve bore. It would be desirable to have all of the pressure sources communicate with the rear chamber through a common bore in order to simplify the valve body.